1. Technical Field
This invention pertains to a runaway current and thermal protection method and apparatus, and more particularly to a method and apparatus for reactivating a system once the cause of the runaway current and excessive heat is eliminated.
2. Background
It has long been known in the prior art to provide thermal shutdown for electronic systems and microchips to prevent the system or chip from generating an excess amount of heat which can damage the system or chip. The generation of excess heat, in one example, is caused when the output load driven by the system is short circuited to ground or low voltage potential which causes the output voltage across the load to drop to substantially zero or the low potential voltage and allow an excess of current to flow through short circuit. This excess current induces a high power dissipation in the system and causes the system to heat up. Protection mechanisms have been developed to prevent the system from continuing to supply the excessive current to the short circuit once the system has reached a predefined temperature. One technique for reactivating a system after thermal shutdown requires the entire system to be powered down and powered up again or re-enabled once the short circuit has been removed. This is time consuming and generally requires user interaction. An alternative technique powers down the system and begins to monitor the temperature of the system. Once the system has cooled below a predefined temperature, the system reactivates. However, if the short is still present, the system will quickly heat up again only to be shut off by the thermal shutdown mechanism of the system. The system will continue to transition on and off, utilizing large amounts of power and potentially damaging the system, until the short circuit is removed.
The present invention provides an apparatus and method for preventing damage to a system due to excessive heat or current and reactivating the system once the cause of the excess current and resulting heat is removed without requiring excessive amounts of power consumption or powering down the system. The apparatus and method is activated when a runaway current condition exists. In one embodiment the shutdown and reactivation apparatus includes a driving circuit which supplies an output current to a load. If the load is short circuited, the output current is greatly increased resulting in a runaway current condition causing the temperature of the system to increase due to high power dissipation. As a result of the increased temperature, the shutdown and reactivation apparatus is activated by triggering a shutdown signal which deactivates the driving circuit such that the output current is no longer supplied to the load. The shutdown signal activates a charging circuit, which supplies a charging current to the load. A first monitoring circuit is coupled with the load, and configured to monitor an output voltage across the load when the shutdown and reactivation apparatus is in the active state. When the short is removed, the charging current is applied to the load to charge a capacitive load. The first monitoring circuit signals the shutdown and reactivation apparatus to transition to a deactivated state when the output voltage across the capacitive load is at least equal to a predefined voltage threshold. The system is re-activated, the charging circuit is deactivated, and the driving circuit again begins to supply the load with the output current when the shutdown and reactivation apparatus is in the deactivated state.
In accordance with the teachings of this invention a novel method and structure is taught which provides the thermal protection of a system while further re-activating the system once the cause of the damaging effects is removed without requiring large power drains, without user interaction, and without requiring the system to be re-enabled or powered down.